open access

https://hdl.handle.net/2115/48356

Hydrogen isotopes in volcanic plumes: Tracers for remote temperature sensing of fumaroles

英語

remote temperature sensing

2011-08-15

Elsevier

Geochimica et Cosmochimica Acta

75(16)

4531-4546

In high-temperature volcanic fumaroles (>400℃), the isotopic composition of molecular hydrogen (H2) reaches equilibrium with that of the fumarolic H2O. In this study, we used this hydrogen isotope exchange equilibrium of fumarolic H2 as a tracer for the remote temperature at volcanic fumaroles. In this remote sensing, we deduced the hydrogen isotopic composition (δD value) of fumarolic H2 from those in the volcanic plume. To ascertain that we can estimate the δD value of fumarolic H2 from those in a volcanic plume, we estimated the values in three fumaroles with outlet temperatures of 630℃ (Tarumae), 203℃ (Kuju), and 107℃ (E-san). For this we measured the concentration and δD value of H2 in each volcanic plume, along with those determined directly at each fumarole. The average and maximum mixing ratios of fumarolic H2 within a plume's total H2 were 97% and 99% (at Tarumae), 89% and 96% (at Kuju), and 97% and 99% (at E-san). We found a linear relationship between the depletion in the δD values of H2, with the reciprocal of H2 concentration. Furthermore, the estimated end-member δD value for each H2-enriched component (-260±3‰ vs. VSMOW in Tarumae, -509±23‰ in Kuju, and -437±14‰ in E-san) coincided well with those observed at each fumarole (-247.0±0.6‰ in Tarumae, -527.7±10.1‰ in Kuju, and -432.1±2.5‰ in E-san). Moreover, the calculated isotopic temperatures at the fumaroles agreed to within 20℃ with the observed outlet temperature at Tarumae and Kuju. We deduced that the δD value of the fumarolic H2 was quenched within the volcanic plume. This enabled us to remotely estimate these in the fumarole, and thus the outlet temperature of fumaroles, at least for those having the outlet temperatures more than 400℃. By applying this methodology to the volcanic plume emitted from the Crater 1 of Mt. Naka-dake (the volcano Aso) where direct measurement on fumaroles was impractical, we estimated that the δD value of the fumarolic H2 to be -172±16‰ and the outlet temperature to be 868±97℃. The remote temperature sensing using hydrogen isotopes developed in this study is widely applicable to many volcanic systems.

学術雑誌論文

Accepted Manuscript

0016-7037